(a) Field of the Invention
The present invention relates generally to optical transmitters for use in high data rate applications. More particularly, it relates to a design and structure for an integrated, laser-based solid state optical transmitter having high data rate capabilities.
(b) Description of Related Art
Fiber optics communications systems are used in a wide variety of applications. In general, fiber optics use a light source to generate and transmit data-carrying modulated light signals through a glass or plastic fiber to a detector. A particularly advantageous light source is a laser.
A known optical transmitter modulation scheme, generally referred to as direct modulation, uses a modulator to modulate directly the laser light source. In direct modulation, one applies an electrical current to the laser that varies with time and carries the digital data information. The current modulates the laser light so that the laser light output has the code of the digital information. However, direct modulation is generally limited by carrier and resonator dynamics (e.g., the device characteristics of the laser itself and the lasing process) to a maximum data rate of about 10-30 Gbps.
An alternative modulation scheme is an external modulation technique wherein the laser output is a constant, and a code is impressed on the laser output by passing it through or reflecting it from a modulator. Known external modulation techniques use edge-emitting lasers or horizontal-cavity lasers wherein the partially reflective mirrors are vertical, thereby creating an optical cavity that is horizontal, and a laser output that exits the laser in a horizontal direction. The edge-emitting laser may be used to feed an edge-coupled modulator in which light is coupled to the modulator at its edge. Light in the edge emitting laser and edge coupled modulator is typically confined in an optical waveguide. Thus, these types of lasers and modulators are also known as guided-wave devices.
Edge-emitting lasers generally involve a trade-off between device performance and the size of the laser output beam. In order to achieve a high quality edge-emitting laser, the size of the output beam is typically very small, e.g., about 1 micrometer in diameter. Thus, it is often difficult to couple efficiently the edge-emitting laser output, via an edge-coupled modulator, to an optical fiber, which is typically 8-10 micrometers in diameter. Accordingly, it is common for only 5-10% of the output laser light actually to go into the fiber unless an optical lens is placed between the modulator and fiber.
The above-described external modulation scheme may be implemented as a single structure in which the laser and the modulator are integrated on the same substrate. Such configurations typically use edge-emitting lasers and edge-coupled modulators (referred to herein as xe2x80x9chorizontal-structure transmittersxe2x80x9d S). However, in addition to the above-described general shortcomings associated with edge-emitting lasers and edge-coupled modulators, these horizontal-structure transmitters suffer from additional shortcomings. For example, the theoretical bandwidth of a horizontal-structure transmitter is limited because to obtain the depth of modulation required for digital communications, there are limits on how small one can make the length of the modulator. These size constraints also inhibit the ability of a designer to reduce the capacitance and the RC time constant of the modulator on the integrated structure. In general, the speed of the modulator is determined by the RC time constant, and the capacitance of the modulator is proportional to the area of the modulator which is the product of its length and width.
Edge-emitting lasers generally have relatively small cross-sectional areas which is the product of its width and height. The near-field image of the light emitted from such lasers typically is small (approximately one micrometer in size) and has an oblong shape. The characteristic is generally unfavorable for either free-space or optical fiber interconnection, and leads to higher insertion loss. Also, edge-emitting typically must be cleaved in order to create the vertical partially reflective mirrors, which can be incompatible with known semiconductor fabrication processes. The ability to implement horizontal-structure transmitters in an array requires many difficult processing steps. Such arrays generally are limited to one transverse direction. Finally, known horizontal-structure transmitters have shown data rates only up to about 40 Gbps.
Thus, there is a need for an optical transmitter that provides high output data rates (up to about 100 Gbps) without increasing cost and manufacturing complexity. There is a further need for an optical transmitter that can be easily constructed in arrays that even further speed up system throughput.
The present invention provides a design and structure for an optical transmitter that achieves high output data rates, while being easy and inexpensive to manufacture. The optical transmitter design of the present invention can be implemented in one and two dimensional arrays without the need for the many and difficult processing steps required to array known optical transmitters.
In a preferred embodiment of the invention, the optical transmitter includes a modulator drive circuit in communication with a modulator, and a laser drive circuit in communication with a laser. The transmitter achieves high output data rates (greater than 10 Gbps) by using an external modulation technique wherein the laser output is a constant, and a code is impressed on the laser output by passing it through a modulator. The modulator outputs modulated light based on laser light received from the laser and modulation control signals received from the modulator drive circuit.
As an example, the modulator drive circuit may be implemented with one or several heterojunction bipolar transistors (HBT), the modulator may be implemented as a surface-coupled, multiple-quantum-well (MQW) modulator, and the laser may be implemented as a vertical cavity surface emitting laser (VCSEL). In operation, incoming radio-frequency (RF) data signals are applied to and amplified by the HBT to drive the MQW modulator. The HBT and the MQW modulator are preferably grown on the same substrate to reduce stray capacitance and inductance. The MQW modulator is illuminated externally by the VCSEL, thereby generating high output optical data rates from the MQW modulator. The VCSEL is integrated and optically aligned with the MQW modulator using self-aligning, flip chip fabrication procedures to reduce optical loss.
Thus, key features of the present invention include using the flip chip process to integrate a surface-emitting VCSEL optically aligned over a surface-coupled modulator, and using mixed device technology to integrate the HBT and the modulator, preferably on the same substrate. The preferred flip-chip procedure includes self-aligning techniques to reduce coupling loss between the VCSEL and the modulator. Using a VCSEL on a separate substrate from the HBT is advantageous because the fastest HBTs have been grown on InP while VCSELs grown on InP have shown poor performance.
The optical cavity of the VCSEL is referred to as vertical because the cavity is parallel to the surface of the VCSEL substrate. Accordingly, the VCSEL light output is in a direction that facilitates its use with the surface-coupled MQW modulator. Thus, the surface-emitting VCSEL and the surface-coupled modulator output are available through the upper and lower horizontal surfaces, respectively, of the VCSEL and the MQW substrates. Modulated light is available from the lower horizontal surface of the transmitter as the output data signal. Constant light is available from the upper horizontal surface of the transmitter and can be used to monitor the average signal power. Having surface-access to the laser and modulator of the disclosed optical transmitter allows many such transmitters to be constructed in an array (one and two dimensional), thereby even further increasing throughput speed.
The present invention may be embodied in an integrated optical transmitter comprising: a drive circuit that receives input data signals and generates drive signals; a laser having a vertical cavity and a laser substrate; a modulator that receives said drive signals and laser light from said laser and generates a modulated optical output corresponding to said drive signals; said drive circuit and said modulator comprising solid state material and having a common substrate distinct from said laser substrate; said laser also comprising solid state material and integrated with said modulator by a flip-chip procedure. The drive circuit could include one or several heterojunction bipolar transistors, the laser could be implemented as a vertical cavity surface emitting laser, and the modulator could be implemented as a multiple quantum well modulator.
In a further embodiment, the flip chip procedure automatically optically aligns the laser with the modulator. The flip chip procedure preferably includes the use of solder bumps to secure the laser in place with respect to said modulator. The transmitter may further include a laser drive circuit also on said common substrate, along with contacts between the laser drive circuit and the solder bumps such that the solder bumps are part of an electrical path that passes laser drive signals from the laser drive circuit to the laser.
The present invention may also be embodied in a high data rate optical transmitter comprising: a modulator drive circuit that receives data input signals and generates modulator drive signals; a laser that has a vertical cavity; a modulator that receives said drive signals and laser light from said laser and generates a modulated optical output; said laser integrated with said modulator by a flip-chip procedure.
In a further embodiment, the flip chip procedure automatically optically aligns the laser with said modulator. The flip chip procedure preferably includes the use of solder bumps to secure the laser in place with respect to the modulator.
In a further embodiment of the above-described transmitter, the transmitter includes a laser drive circuit, along with contacts between the laser drive circuit and the solder bumps such that the solder bumps are part of an electrical path that passes laser drive signals from the laser drive circuit to the laser.
The present invention may also be embodied in a method of making an integrated transmitter comprising the steps of: providing a modulator drive circuit and a modulator in communication therewith; and integrating a laser with said modulator using a flip chip procedure, wherein said laser comprises a vertical cavity.
In a further embodiment of the above-described method, the flip chip procedure automatically optically aligns the laser with the modulator. The flip chip procedure preferably includes the use of solder bumps to secure the laser in place with respect to the modulator.
In an even further embodiment of the above-described method, the method further includes the steps of: providing a laser drive circuit; and providing contacts between the laser drive circuit and the solder bumps such that the solder bumps are part of an electrical path that passes laser drive signals from the laser drive circuit to the laser.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.